This thesis describes three experiments in which low temperature photoluminescence or selective excitation luminescence was used as a probe of shallow electronic states in three different semiconductors.
Chapter 1 serves as an introduction to the other chapters. It begins with a short review of some of the theory used to describe the electronic states studied in these experiments. After a brief mention of band theory, effective mass theory for states weakly bound to charged impurity centers is reviewed. Free excitons are briefly described, as is the binding of free excitons to impurities to form bound exciton and bound multi-exciton states. This is followed by a discussion of the interaction of the electronic states with light. Light is used both to create electron-hole pairs, and is emitted when the pairs subsequently recombine. The recombination occurs after the electrons and holes have been captured into the bound states, or have relaxed to band states near the band minima in energy. The energy and lineshape of the emitted light are characteristic of the nature of the states, and in some cases the identity of the centers involved. The chapter is closed with a summary of the results of the experiments described in the other three chapters.
Chapter 2 describes a photoluminescence study of the alloy semiconductor Hg 1-xCdxTe. This alloy is disordered in the sense that the cation site is randomly occupied by either the Hg or the Cd, but it retains the structural order of perfect crystals. Band-to-band, band-to-acceptor, and donor-acceptor luminescence were observed in material of x = 0.32 and 0.48, and bound exciton recombination luminescence was observed in material of x = 0.48. The band-to-band lineshape and variation in intensity with pump power are appropriate to electron-hole plasma with recombination proceeding without wavevector conservation. From lineshape separations, acceptor binding energies of 14.0 ± 1.0 and 15.5 ± 2.0 meV were estimated for x = 0.32 and 0.48 material. respectively. Donor binding energies were estimated to be 1.0 ± 1.0 and 4.5 ± 2.0 meV, respectively. The observation of the bound exciton recombination luminescence in x = 0.48 material is consistent with Osbourn and Smith's work indicating radiative decay for the bound exciton should dominate non-radiative, Auger decay in material with x > 0.4. Shifts in luminescence energy across the sample imply a change in composition across the sample of 0.03 cm-1.
The transient decay of bound multi-exciton complex lines in the photoluminescence spectrum of Si:P is reported in Chapter 3. These measurements were made to test a shell model for the multiple particle states. This model makes some predictions about the origin of the observed lines. One of these predictions is that the mth. member of the β series of lines is produced when a complex with m + 1 excitons decays to form the ground state of a complex with m excitons. The αm+i line, in this model, is formed by the decay of the same complex, but into an excited state of the complex with m excitons. As a test of the model, the decays of the α lines are compared to the decays of the β lines to determine whether those transitions predicted to have common initial states decayed in an identical fashion. The results are consistent with β2 having the same initial state as α3. The results for β1 and β3 are less certain. Their decays are consistent with β1 having the same initial state as either α2 or α3, and with β3 having the same initial state as α4. Overlap between lines, uncertainty in the decay of the boron bound exciton and the presence of unidentified background make it difficult to make conclusive line assignments. However, the results are consistent with a shell model for the states, and with more recent theoretical calculations of the level structure.
Chapter 4 describes the measurement of the excited state levels of two different shallow acceptors in bulk-grown GaAs. The technique used to make the measurements was selective excitation luminescence. Jn this technique, the energy difference between the laser light used for excitation and the resulting luminescence yields the acceptor excited state energy. The 1S - 2S energy differences for the two samples were measured to be 21.5 and 18.5 meV, respectively. By comparing these values to those measured by two-hole transition luminescence in high quality epitaxial GaAs, the acceptors were identified as Zn and C. The measured 1 S - 2P energy differences also support the identification. These studies indicate that selective excitation luminescence can be used to study shallow impurity states in heavily doped semiconductors. In particular, selective excitation luminescence can be used to identify shallow acceptors in bulk-grown GaAs, and hence can be used as a diagnostic tool for bulk-grown samples.
